The present invention relates to steel sheet for porcelain enameling, method for producing the same, porcelain enamel products and a method for producing the same. In further detail, the invention relates to steel sheet for porcelain enameling having excellent adhesion with enamel and a method for producing the same, as well as to a porcelain enamel product and a method for producing the same, which enables excellent enamel adhesion by xe2x80x9cdirect-onxe2x80x9d enameling of cover coat enamel on Ti-added steel sheet.
Porcelain enamel products are widely used as kitchen and table-top articles, components of heating appliance and components of cooking appliance, bathtubs, interior and exterior materials of buildings, and the like. Enamel products are generally produced by firing twice; a ground coat enamel is first applied on the steel sheet and fired, a cover coat enamel is further applied thereon, and fired again. In order to reduce the production cost, however, a production method of direct enameling for only once, i.e., xe2x80x9cdirect-on enamelingxe2x80x9d, comprising providing the cover coat enamel directly on the steel sheet followed by firing is employed. However, in the production method of direct-on enameling, it is necessary to perform pretreatment oh the steel sheet before enameling, such as intense pickling and Ni dipping treatment. Further, as a steel sheet for obtaining favorable adhesion of the enamel product with the steel sheet, a rolled sheet of high oxygen steel containing oxygen at a high concentration, which is produced by reducing C content in the steel-making stage and produced by continuous casting without performing deoxidation treatment, is widely utilized. However, a high oxygen steel generally suffers poor workability, and its application to usages requiring severe processing is limited.
Japanese Patent Publication No. 24413/1979 discloses that, by coating the surface of a steel material with an alloy of one or two types of metals selected from Ni and Fe with one or two types of metals selected from Mo and W, excellent affinity with the cover coat enamel and adhesion with the cover coat enamel can be obtained. However, there is no description concerning the workability of the steel sheet.
On the other hand, in usages requiring severe workability such as kitchen wares or bathtubs, rolled sheets of Ti-added steel or B-added steel have been used. However, as is disclosed in Japanese Patent Laid-Open No. 140286/1998, although Ti-added steel sheet is superior in workability, black specks defects generate in the enamel layer in case direct-on enameling is performed. Accordingly, the enamel layer had to be formed by means of ground coat enamel finishing or by applying the cover coat and ground enameling in two times.
As described above, there is required a steel sheet for porcelain enameling by reduced production steps and energy consumption, thereby reduced in production cost, yet improved in workability and having an enamel layer with high adhesion even in case direct-on enameling is performed only once.
In the present invention, the objects are to provide steel sheet for porcelain enameling having excellent adhesion with the steel sheet by applying direct-on enameling of cover coat enamel only once and still free of black specks defects, to provide the method for producing the same, as well as to provide a porcelain enamel product and the method for producing the same, in which Ti-added steel sheet is used.
The steel sheet for porcelain enameling according to present invention that solves the problems above is characterized by that it comprises a Ti-added steel sheet containing 0.01% by weight (wherein, % represents xe2x80x9c% by weightxe2x80x9d hereinafter) or less of C, 0.5% or less of Mn, 0.04% or less of P, 0.04% or less of S, 0.01 to 0.50% of Ti, and balance Fe accompanied by unavoidable impurities, which is obtained by providing thereon a Nixe2x80x94Mo alloy plating film, and which is then subjected to heat treatment.
The reason for confining each of the components in the steel sheet for porcelain enameling above according to the present invention is as follows.
[C]
From the viewpoint of suppressing the generation of pores and black specks on firing the enamel while assuring favorable workability, the content of C is set to 0.01% or less.
[Mn]
Manganese bonds with S to exhibit effects of suppressing cracking attributed to embrittlement from occurring during hot working; hence, Mn should be incorporated at a concentration of 0.5% or lower. In case the content of Mn exceeds 0.5%, the content of sulfides in the steel decreases as to reduce the degree of surface roughening of the steel sheet during pickling, and it results in a loss of anchoring effect. This leads to a drop in adhesion of enamel.
[P]
Although P improves adhesion, from the viewpoint of suppressing the generation of pores and black specks on firing the enamel, the content thereof is set to 0.04% or lower.
[S]
Sulfur accelerates surface roughening of the steel sheet on pickling as to improve the adhesion of the enamel by anchoring effect. However, S causes cracks due to embrittlement on hot working. Hence, the content of S is constrained to 0.04% or lower.
[Ti]
Titanium should be contained in a range of from 0.01 to 0.50%. In case the content falls outside this range, hardening occurs as to impair moldability.
The steel sheet for porcelain enameling according to the one embodiment of the present invention is characterized by that, in the steel sheet for porcelain enameling the content of elements present in the surface of the steel sheet as measured by an energy-dispersion type X-ray microanalyzer hereinafter referred to as xe2x80x9cEDXxe2x80x9d is 5 to 75% Ni, 3 to 40% Mo, and 5 to 82% Fe provided that Ni, Mo, and Fe in total is 100%.
Furthermore, the method for producing steel sheet for porcelain enameling according to the one embodiment of the present invention is characterized by that it comprises providing a Ni-Mo alloy plating on the steel sheet described above, followed by applying a heat treatment thereto. Further, the method for steel sheet for porcelain enameling according to one embodiment of the present invention is characterized by that, as the Nixe2x80x94Mo alloy plating, the plating is performed in such a manner that the plating film contains Ni at a coverage of 1.5 to 20.0 g/m2 and Mo at a coverage of 0.4 to 7.0 g/m2. Furthermore, the method for steel sheet for porcelain enameling according to one embodiment of the present invention is characterized by that the heat treatment is performed in a temperature range of from 500 to 900xc2x0 C.
Moreover, the porcelain enameled product according to one embodiment of the present invention is characterized by that it comprises an enamel layer provided on one of the steel sheets for porcelain enameling as described above. Then, the method for producing a porcelain enameled product according to one embodiment of the present invention is characterized by that it comprises once applying a cover coat enamel on one of the steel sheets for porcelain enameling as described above, followed by applying firing thereto.
The mode for carrying out the present invention is described below.
The present invention is based on the findings that, by applying a Nixe2x80x94Mo alloy plating on a Ti-added steel sheet having a specified composition range and containing ultra-low carbon, and by applying heat treatment thereto in order to control the content of Ni, Mo, and Fe present in the surface of the steel sheet for porcelain enameling in a predetermined range, excellent enamel adhesion properties are obtained even on porcelain enameled products having the enamel provided by direct-on enameling for only once.
The Ti-added steel sheet containing low carbon for use in the present invention has its composition adjusted as such that it should contain 0.01% by weight or less of C, 0.5% or less of Mn, 0.04% or less of P, 0.04% or less of S, 0.01 to 0.50% of Ti, and balance Fe accompanied by unavoidable impurities. A slab is then produced from the steel having its composition adjusted to the range above by means of continuous casting. The resulting slab is then hot rolled, or, hot rolled after re-heating. Then, after pickling and descaling by a known method such as sulfuric acid pickling and the like, the resulting product is cold rolled at a draught of about 50 to 95%, annealed at a temperature of recrystallization temperature or higher but lower than the Ac3 point by means of core box annealing or by continuous annealing process, and subjected to refining rolling at a draught of about 0.1 to 5% to obtain the steel sheet for use in the present invention.
Subsequently, a Nixe2x80x94Mo alloy plating is provided to the resulting steel sheet. The alloy plating may be performed by either means of electroless plating or electrolytic plating, but from the ease of controlling the alloy composition, preferred is electrolytic plating. As the plating bath, there is used an aqueous solution having supplied thereto Ni ions in the form of a salt of an inorganic acid, such as a sulfate, a nitrate, a halide, etc. and Mo ions in the form of an ammonium salt of a metallic acid salt and the like, having further added thereto, as a complexing agent, an organic acid such as citric acid, tartaric acid, or a malic acid or a salt thereof. Then, after adding an acid or an alkali to the resulting aqueous solution to control the pH in a range of from 2 to 4, and adjusting the bath temperature in a range of from 30 to 50xc2x0 C., direct current electrolysis is performed at a current density of from 5 to 30 A/dm2 by using a Ni plate as the anode. Preferably, degreasing treatment and pickling treatment are performed on the steel sheet by an ordinary method just before applying the plating above, because the steel sheet before plating tends to be oxidized with passage of time or be brought into contact with oils and fats.
Concerning the components in the coating formed by alloy plating above, the coating contains 1.5 to 20.0 g/m2, preferably 2.0 to 6.0 g/m2 of Ni, and 0.4 to 7.0 g/m2, preferably 1.0 to 2.0 g/m2 of Mo. The content can be obtained by fluorescent X-ray spectroscopy. In case the content of Ni and Mo in the plating should fall outside the range above, favorable adhesion cannot be assured between the enamel and the steel sheet, because the elements Fe, Ni, and Mo in the surface of the steel sheet cannot be pertained in the preferred range on applying heat treatment after plating as described below.
Then, heat treatment is performed after providing the Nixe2x80x94Mo alloy plating on the steel sheet as described above. The heat treatment is carried out in a manner similar to annealing generally performed on an ordinary steel sheet. As the heat treatment, i.e., as the annealing, there can be used either core box annealing or continuous annealing without any problem. Although depending on the content of Fe, Ni, and Mo pertained on the surface of the steel sheet after heat treatment, the conditions of annealing are, heating in the temperature range of from 500 to 900xc2x0 C. for a time duration of from 1 minute to 15 hours under a reducing atmosphere of a gaseous decomposed ammonia, more preferably, heating in the temperature range of from 550 to 750xc2x0 C. for a time duration of from 1 to 8 hours is performed.
In the manner described above, steel sheet for porcelain enameling according to the present invention can be obtained. By the heat treatment described above, Ni and Mo diffuse into the steel sheet, and Ni and Mo undergo mutual diffusion as to change the content of Fe, Ni, and Mo present in the surface of the steel sheet. The content of the elements present in the surface of the steel sheet can be measured by using a surface analyzer of, for instance, EDX. After the heat treatment, the content of Fe, Ni, and Mo present in the surface of the steel sheet is 5 to 75% Ni, 3 to 40% Mo, and 5 to 82% Fe; preferably, 8 to 50% Ni, 5 to 25% Mo, and 35 to 80% Fe, and by adjusting the content of Ni, Mo, and Fe in such a manner that the total thereof should become 100%, an excellent adhesion properties of enamel with the steel sheet can be obtained. That is, by thus performing heat treatment after applying Nixe2x80x94Mo alloy plating, a Moxe2x80x94Ni layer containing Fe at a certain concentration or higher is formed on the surface of the plated steel sheet, and thereby a favorable enamel adhesion can be assured.
The steel sheet for porcelain enameling thus obtained as described above can be enameled as it is in the state of flat plate without processing, or may be enameled after shaping and processing it into the desired shape by applying bending and drawing. In general, porcelain enameling is performed by applying, as the ground coat enamel for assuring adhesion, an enamel containing Ni and Co; for instance, a ground enamel containing 15 to 20% of Na2O3 and K2O, 3 to 6% of CaF2, 3 to 6% of Al2O3, 13 to 18% of B2O3, 50 to 55% of SiO2, and 0.3 to 1.5% of CoO and NiO. Then, a cover coat enamel having beautiful appearance is applied. In case of using the steel sheet for porcelain enameling according to the present invention, not only the commonly employed base enamel containing Ni and Co, but also a cover coat enamel free from Ni and Co, for instance, a cover coat enamel generally used in the twice enameling method containing 10 to 15% of Na2O3 and K2O, 1 to 4% of CaF2, 0 to 3% of Al2O3, 7 to 13% of B2O3, 48 to 50% of SiO2, 0 to 2% of MgO and ZnO, and 15 to 20% of TiO2, may be applied by direct-on enameling to obtain excellent enamel adhesion. The condition of enameling is such that, after applying the enamel at a dry thickness in a range of from 80 to 300 xcexcm, firing is performed by heating in air in the temperature range of from 700 to 900xc2x0 C. for a time duration of from 1 to 5 minutes. In this manner, a porcelain enamel product of the present invention can be obtained.